Modern fuel delivery systems for automotive vehicles with engines having fuel injectors have utilized an electrically driven fuel pump in the vehicle main fuel tank. Typically, the electric fuel pump is mounted either directly in the vehicle tank, or is mounted within a reservoir canister received in the tank as shown for example in U.S. Pat. Nos. 4,747,388; 4,807,582; 4,831,990; 4,893,647; and 4,964,787. The canister reservoir supplies fuel to the pump in the event there is an interruption in the availability of fuel from the tank, such as when, under low fuel level conditions, cornering of the vehicle causes sloshing or movement of the fuel away from the pump inlet and to one side or the other of the tank, and/or when the tank is excessively tilted by vehicle inclination on severe grades, or when essentially all of the fuel in the main tank has been consumed or used. Typically, the output of the fuel pump is greater than that required by the vehicle engine and the excess fuel is either returned from the fuel injectors to the tank or to the in-tank canister reservoir.
Typically, in a no-return fuel system, there is only one fuel supply line between the fuel pump module and an engine fuel rail or manifold distributing fuel to the individual fuel injectors, and downstream of the fuel injectors there is no line returning unused fuel from the rail or manifold to the fuel tank. In such non-return fuel systems excess fuel is bypassed directly to the tank or canister reservoir, typically by a pressure regulator usually located closely downstream of the pump outlet within the tank or canister, or by a return line to the tank or canister when the regulator is exteriorly remote from the tank.
In the aforementioned fuel systems in which the fuel pump is mounted in a fuel canister special valving has been provided between the canister, reservoir and the pump inlet to supply reserve fuel when the main tank supply is low; see for example the above cited U.S. Pat. No. 4,747,388. In this system, a valve actuated by pump suction lift of the primary fuel filter opens to provide fuel from the canister reservoir to the pump inlet when the pump inlet is starved because of low fuel or because of movement of fuel in the main tank to one side or the other during vehicle negotiation of a curve in the road. Further examples of use of a reservoir canister and special valving for supplying reserve fuel to a fuel pump are disclosed in U.S. Pat. Nos. 4,546,750 and 5,237,977. In these systems, under conditions of low fuel in the main fuel tank a valve associated with the canister responds to inertial forces created by side swerving motion of the vehicle, and/or in response to gravitational forces caused by excessive tilting of the vehicle on hills and grades, to open and allow flow of fuel from the canister to the fuel flow passage leading to the pump to thereby prevent starving of the engine due to no or low pump inflow from the main fuel tank.
Another system in accordance with the present invention for preventing starving of the fuel pump and hence the engine due to low flow from the main tank is that set forth in co-pending benefit parent application Ser. No. 08/496,950, filed Jun. 30, 1995 in the name of Charles H. Tuckey and assigned to Walbro Corporation, which is incorporated herein by reference. In this system a fuel pump is mounted directly in the tank and an in-tank standpipe reservoir is also mounted in the tank alongside of the pump and receives bypass fuel from the pump. A restricted orifice provides the standpipe outlet and feeds into the space above the filter media of the primary fuel filter to thereby provide an always-open reserve fuel flow path from the standpipe reservoir to the fuel pump inlet. The orifice is calibrated to be balanced with pump draw to prevent standpipe overflow so most if not all bypass fuel is returned to the pump inlet rather than overflowing to the main tank. This system also enables the reservoir fuel at pump shut down to gravity flow back into the main tank from the interior of the main filter under low tank fuel level conditions to thereby backflush contaminants from the exterior surface of the main filter.
Generally speaking two types of electrically driven rotary fuel pumps have hitherto been used in the vehicle main fuel tank for pumping liquid fuels to the fuel injectors of the automotive vehicle engine, namely a turbine type vane pump or a positive displacement pump. One preferred form of a positive displacement gear rotor type electric fuel pump is disclosed in U.S. Pat. No. 4,697,995, and a suitable turbine regenerative fuel pump is disclosed in U.S. Pat. No. 5,257,916, the disclosures of which are incorporated herein by reference.
In-tank vehicle fuel pumps must be capable of operating in a wide range of ambient temperatures. The hydrocarbon fuels (gasoline and alcohol) have a relatively low boiling point. In certain geographical areas, the ambient temperatures may reach 110.degree. to 120.degree. Fahrenheit. The temperature in the fuel tank below the automotive vehicle may be even higher than this. Since these pumps are frequently mounted in the fuel tanks, there is a great likelihood that the fuel in the pump may vaporize. The pumps when mounted in reservoir canisters are usually positive displacement pumps and it is necessary that the entry to the pump chambers create a low pressure to draw fuel into the pumping chambers. This reduced pressure alone may cause a change in state of the fuel from liquid to vapor at elevated temperatures and significantly reduce the efficiency of the pump.
In another condition as, for example, when a vehicle has been operating and then the engine shut off for a period, the fuel line between the pump and fuel injectors full of liquid fuel under pressure whereas the fuel in the pump can be completely vaporized due to the elevated temperature in the fuel tank and pump itself. Thus, when the engine is restarted, the pump is full of vapor and even the fuel in the entrance filter may be vaporized. The pump cannot, under these conditions, generate enough pressure to move the fuel in the pressurized fuel supply line.
Accordingly, both of the aforementioned U.S. Pat. Nos. 4,697,995 and 5,257,916 patents disclose a rotary pump construction with a built-in vapor purging system which will enable the pump to operate under the conditions above described without interruption of the fuel supply, with one major exception. In accordance with the present invention it has been found that turbine vane pump when disposed within an in-tank canister reservoir does not operate satisfactorily to sufficiently purge itself of vapor under adverse temperature or other vapor-inducing conditions. Due to the preferred location and size of the vapor purge port in the first or pre-channel zone and the inherent operating characteristics of the turbine pump, only a small amount of vapor pressure build up can be produced by such a pump at the pump vapor purge port. Hence, unlike the positive displacement pumps employed within the in-tank canister reservoirs as described previously, it has been found that such a vane type pump cannot purge itself of vapor in the pumping channel if the pressure differential between the pump inlet and the vapor port outlet exceeds about 21/2 inches of water.
This constraint as to self-purging of vapor does not apply to positive displacement gear rotor pumps of the aforementioned type since they characteristically pump both fuel vapor and air very well. Hence, when mounted inside an in-tank fuel reservoir canister and provided with a vapor purging port communicating with the column of fuel in the canister and located at approximately the elevation of the rotary pump chamber, such positive displacement pumps can purge vapor into the canister against a gravity fuel differential pressure head ranging from say six on up to ten or twelve inches, as when the canister reservoir is full and the gravity head at the pump inlet exerted by the body of fuel in the tank exterior to the canister reservoir is very low.
Nevertheless, some OEM automotive vehicle manufacturers have preferred turbine vane fuel pumps for use in fuel tanks over gear rotor positive displacement pumps for a variety of reasons, even though they have had to forsake the advantages of a canister reservoir-type reserve supply of fuel for preventing starvation of fuel supply to the turbine pump from the main tank in order to obtain vapor self-purging operation. In an attempt to make up for this lack of the preferred selectable alternative canister reserve supply of fuel, such as that provided in the aforementioned U.S. Pat. No. 4,747,388, some automotive manufacturers have mounted turbine vane fuel pumps in specially configured fuel tanks with the pump inlet submerged in a so-called "swirl pot", i.e., a molded-in basin in the tank bottom in an attempt to maintain an adequate body of fuel in the vicinity of the fuel pump inlet to prevent pump starvation. However such swirl pots are still subject to being emptied by severe vehicle cornering or excessive vehicle inclination, as well as when essentially all of the fuel of the main tank has been consumed or used. Such swirl pots also inherently must be limited in their length, width and depth dimensions, thereby imposing a further constraint on the bulk of the fuel pump and associated filter package which can be accommodated in such swirl pots.
Despite such hitherto prevailing limitations, some OEM automotive vehicle manufacturers have preferred to use turbine vane-type fuel pumps over gear rotor positive displacement pumps for in-tank mounting because such turbine vane pumps generally are quieter and smoother running, and also have low tank fuel level operation performance characteristics preferred by such vehicle manufacturers over those of gear rotor positive displacement pumps. With a typical positive displacement gear rotor fuel pump, such as that disclosed in the aforementioned U.S. Pat. No. 5,257,916, when the pump encounters like conditions with a large presence of vapor in the pump chamber, the positive displacement pump will continue to pump liquid fuel but will also pump vapor as well. When this condition occurs the fuel pumped to the engine contains vapor and/or air entrained with liquid fuel. This will cause the vehicle engine to spit and stumble or otherwise run rough even though the engine continues to run on this vapor and liquid fuel mixture. However, because of constraints imposed by engine control unit (ECU) operational characteristics and exhaust emission requirements, several vehicle manufacturers would prefer to see the instantaneous shut off characteristic under extreme low tank fuel level conditions of the turbine vane fuel pump, wherein the pump ceases pumping any fuel.